The present invention relates to a high NA (numerical aperture) objective lens installed in an optical pick-up of an optical disc apparatus that is capable of using a plurality of kinds of optical discs whose cover layers are different in thickness. Particularly, the present invention relates to the objective lens that has a diffractive lens structure formed on a surface of a refractive lens.
The optical disc includes an information layer on which digital information is recorded, and a transparent cover layer that covers the information layer.
There are several types of the optical discs. A CD (compact disc) or a CD-R (CD recordable) has the cover layer whose thickness is 1.2 mm, and the thickness of the cover layer of a DVD (digital versatile disc) is 0.6 mm.
Such a difference of thickness of the cover layer changes the relative position of the information layer with respect to a turntable, i.e., the distance between the optical pick-up and the information layer. Namely, the thicker the cover layer is, the greater the distance to the information layer from the optical pickup is. For example, the optical pick-up is required to move a beam spot away from the optical pick-up by 0.6 mm in the cover layer, which is equivalent to 0.4 mm in air, when the DVD is replaced with a CD or a CD-R.
Although a paraxial beam spot moves as the objective lens is moved, the change of the thickness of the cover layer changes spherical aberration. If the optical pick-up moves only the objective lens when the disc is replaced, wavefront of the laser beam is deteriorated. For instance, when the objective lens, which is designed to minimize the spherical aberration for the DVD, is used for reproducing the Information from the CD, the spherical aberration becomes too large to reproduce the information even if the objective lens moves to bring the beam spot to be coincident with the information layer.
Therefore, there has been known as prior art, an optical pick-up that adjusts the condition of the laser beam entering into the objective lens depending upon the thickness of the cover layer.
For example, Japanese Provisional Patent Publication No. Hei 7-98431 discloses such an optical pick-up. The optical system shown in this publication employs a holographic lens on the laser source side of the objective lens to divide the laser beam from the laser source into a zero-order parallel diffracted beam and a first-order divergent diffracted beam. The zero-order diffracted beam is used for the optical disc having a thinner cover layer (i.e., a DVD), the first-order diffracted beam is used for the optical disc having a thicker cover layer (i.e., a CD and a CD-R). The optical pick-up disclosed in the publication enables formation of diffraction-limited beam spots for the respective optical discs.
However, since the optical pick-up disclosed in the publication always divides the laser beam from the laser source into the zero- and first-order diffracted beams, and only one of these beams is used for recording/reproducing information at a time, the maximum efficiency in use of the light quantity is not more than 40%.
Further, since the diffracted beam of only one diffraction order is being used for recording/reproducing the information, the diffracted beam of the other diffraction order is an unnecessary beam. When the unnecessary order diffracted beam is not adequately diffused on the information layer of the optical disc, a light quantity of the unnecessary order diffracted beam reflected from the optical disc varies in response to the recorded information on the optical disc, which causes noise in the reproducing signal.
Still further, the recording density of a DVD is higher than that of a CD, which requires the optical pick-up for a DVD to form a smaller beam spot than the optical pick-up designed for the exclusive use for a CD (hereinafter referred as an exclusive CD pick-up). Since the diameter of the beam spot has a positive correlation with the wavelength of the laser beam, the optical pick-up for a DVD requires the laser source whose oscillation wavelength is 635 through 660 nm that is shorter than the oscillation wavelength of an exclusive CD pick-up (i.e., 780 through 830 nm). On the other hand, the reflection characteristics of a CD-R require the laser source whose oscillation wavelength is about 780 nm.
Accordingly, when the optical pick-up having a single laser source as described in the publication employs a laser source that emits a laser beam having a shorter oscillation wavelength, it cannot reproduce the information from a CD-R.
It is therefore an object of the present invention to provide an objective lens for an optical pick-up, which is capable of recording/reproducing information on a plurality of kinds of the optical discs (e.g., CD, CD-R and DVD) whose cover layers are different in thickness. Further, the present invention is aimed to provide the objective lens that has higher efficiency in use of the light quantity than the conventional objective lens as disclosed in the above-identified publication.
For the above object, according to the present invention, there is provided an improved objective lens for an optical pick-up, which includes a refractive lens having a positive refractive power, and a diffractive lens structure having a plurality of concentric ring-shaped steps that are formed on at least one lens surface of the refractive lens. The diffractive lens structure has such a wavelength dependence that at least two diffracted beams having different wavelengths with the same diffraction order form appropriate wavefronts for at least two kinds of optical discs having cover layers of different thickness. Further, the diffractive lens structure has a predetermined power at any points in a middle ring area that is concentrically arranged about the optical axis at a middle range of the radius of the refractive lens. In the other words, the diffractive lens structure has no point where a power is zero. With respect to a lens, the power means the reciprocal of its focal length.
With this construction, the diffracted beam of the predetermined diffraction order at the first wavelength forms an appropriate beam spot on the recording layer of the first optical disc and the diffracted beam of the same diffraction order at the second wavelength forms an appropriate beam spot on the recording layer of the second optical disc.
When the diffractive lens structure has the power, the convergence of the diffracted beam varies with diffraction order, which diffuses the unnecessary order diffracted beam. Further, the light beam passing through the middle ring area has a great influence on forming a beam spot. Therefore, in the invention, since the diffractive lens structure has a predetermined power in the middle ring area, the unnecessary order diffracted beams are adequately diffused while the beam spot is clearly formed by the necessary order diffracted beam, which can reduce the noise in a reproduced signal.
An additional optical path length added by the ring-shaped steps of the diffractive lens structure is expressed by an optical path difference function "PHgr"(h) as follows:
"PHgr"(h)=(P2h2+P4h4+P6h6+ . . . )xc3x97mxc3x97xcex
where P2, P4 and P6 are coefficients of second, forth and sixth orders, h is a height from the optical axis, m is diffraction order and xcex is wavelength.
According to the invention, the optical path difference function "PHgr"(h) is defined such that the value thereof monotonously varies in the middle ring area without taking the extreme value. This means that the diffractive lens structure has a predetermined power at any points in the middle ring area.
The lens surface of the refractive lens is divided into a high NA exclusive area through which a light beam of a high NA, which is necessary only for an optical disc having a high recording density, passes, and a common area through which a light beam of a low NA, which is necessary and sufficient for an optical disc having a low recording density, passes.
The diffractive lens structure may be formed only within the common area, or over the entire area including the common area and the high NA exclusive area.
The middle ring area is preferably included in the common area. In this case, the diffractive lens structure may have the power at any points within the common area, i.e., the optical path difference function "PHgr"(h) may be defined such that the value thereof monotonously varies in the common area. On the other hand, the diffractive lens structure may have point where a power is zero, i.e., the optical path difference function "PHgr"(h) may take extreme value, in an area that surrounds the middle ring area.
It is preferable that the common area is inside of the circle where a light beam whose NA is 0.45 through 0.50 passes, and the middle ring area is defined as an area through which a light beam whose NA is 0.20 through 0.38 passes.
Further, the diffractive lens structure is preferable to have such a wavelength dependence that spherical aberration varies in the undercorrected direction as wavelength of incident light beam increases. As described above, the spherical aberration varies in the over corrected direction as the thickness of the cover layer increases. Therefore, when a longer wavelength laser source is used for an optical disc having a thicker cover layer, and a shorter wavelength laser source is used for an optical disc having a thinner cover layer, the change of the spherical aberration due to change of the cover layer""s thickness is corrected by the above-mentioned wavelength dependence of the diffractive lens structure.
The diffractive lens structure is preferably satisfy the condition (1) or (2);
P2xc3x97(h45)2xc3x97m less than 3xe2x80x83xe2x80x83(1)
P2xc3x97(h45)2xc3x97m greater than 8xe2x80x83xe2x80x83(2)
where h45 is the height from the optical axis of a point where a light ray whose NA is 0.45 at the wavelength corresponding to the optical disc having thicker cover layer intersects the diffractive lens structure.
Further, when the condition (1) is satisfied, the following condition (3) is preferably satisfied, when the condition (2) is satisfied, the following condition (4) is preferably satisfied;
"PHgr"(h45)/xcex less than xe2x88x927xe2x80x83xe2x80x83(3)
"PHgr"(h45)/xcex greater than 8.xe2x80x83xe2x80x83(4)
Still further, when the condition (1) is satisfied, it is preferable that the diffractive lens structure satisfies the following condition (5) over the entire area on the lens surface;
"PHgr"(h) less than 0.5xcex.xe2x80x83xe2x80x83(5)